This invention relates to a method and apparatus useful in making containers in the form of tubular members fashioned from thermoplastic coated paperboard.
Containers fashioned from paperboard are often formed from single blanks having thermoplastic coatings, of the same or of different thicknesses, on their surfaces. Examples of such containers are afforded by paperboard cans and by the well known gable-top milk cartons. Other containers of paperboard may be circular, oval or of any other desired cross-section. While many paperboard containers are fashioned from a single blank, often the side walls are formed with separate formations or operations being required to form the bottom and the top of the container. Whether fashioned from a one piece blank or from several elements, the side walls of most paperboard containers require a side seam. This seam is formed by heating opposite edges of the blank, one surface each of these opposite edges being softened and becoming tacky so that when overlapped and pressed together the thermoplastic will form a joint, thereby forming the seam. Presently employed techniques for heating the edges of the blank which are to form the lapped joint or seam employ heating by gas, by hot air, by hot rollers, or by ultrasonics. Each of these techniques exhibits various deficiencies such as inconsistent or nonuniform heating, inefficient utilization of the energy to soften the plastic, or slow process speeds.
The use of laser energy for communications, for cutting metals, and also for sealing and cutting plastic materials is known. However, lasers have not been widely applied in the paper container field. Workers in this art have apparently not recognized that laser energy may be employed in paperboard containers such as for forming seams in the bodies of such containers. By the practice of this invention, relatively high line speeds of paperboard blanks used to form containers may be realized. For example, the practice of this invention admits of line speeds up to 20,000 feet per hour. Further, because of the uniformity of the applied energy from the laser beams, the resultant side seam of the container wall is of uniform quality.